Research has been performed on a system to achieve the improvement of the economical efficiency of a whole recycling system in which uranium and plutonium are recovered by utilizing a molten salt electrolytic technique as a reprocessing technique for recycling of spent nuclear fuels used in nuclear reactors. The molten salt electrolytic technique is expected to be high in economical efficiency. (See, for example, Japanese Patent Laid-Open Specification No. 2001-141879.) The relevant electrolytic techniques include an oxide electrowinning method and a metal electrorefining method. When the chemical forms of uranium and plutonium in the electrodeposit are oxides, the oxide electrowinning method is employed.
The oxide electrowinning method is to recover oxides of uranium and plutonium through a simultaneous electrolytic step, a dissolution step by chlorination and a MOX recovery step. In this method, the spent nuclear fuel is first placed in the bottom portion of a crucible doubling as an anode, and then electrolysis is carried out between the anode and a cathode installed in an upper portion of the crucible. By this operation, uranium oxide contained in a large amount in the spent nuclear fuel is dissolved into the molten salt due to anodic oxidation, and simultaneously recovered by depositing uranium oxide on the surface of the cathode due to cathodic reduction (a simultaneous electrolytic step). Thereafter, the electrolytic operation is stopped, and uranium oxide, plutonium oxide and other elements remaining in the spent nuclear fuel are dissolved into the molten salt by blowing chlorine gas into the molten salt to convert them to chlorides thereof (a dissolution step by chlorination). After the whole spent nuclear fuel has been dissolved into the molten salt, electrolysis is carried out between the anode doubling as the crucible and the cathode installed in the upper portion of the crucible, and the oxides of uranium and plutonium are recovered by depositing the oxides in a mixed state on the surface of the cathode (a MOX recovery step).
The reactions involved in the respective steps are shown below:
a simultaneous electrolytic step:UO2→UO22+ (anodic reaction)UO22+→UO2 (cathodic reaction)
a dissolution step by chlorination:UO2+Cl2→UO2Cl2PuO2+C+2Cl2→PuCl4+CO2
a MOX recovery step:UO2Cl2→UO2+Cl2 (cathodic reaction)PuCl4+O2→PuO2+2Cl2 (cathodic reaction)
As described above, in the conventional technique, a constitution is adopted such that the crucible containing the substance to be treated doubles as the anode, the cathode is installed in the molten salt, and electrolysis is carried out between the anode (the crucible) and the cathode. Alternatively, there is another constitution such that the anode and cathode are installed in the crucible and electrolysis is carried out therebetween.
However, such a conventional technique as described above has suffered from the following problems to be solved. When the crucible doubles as the anode, in the steps other than the simultaneous electrolytic step, the distance between the anode and the cathode is uniformly maintained to be uniform and hence the current density is uniform, so that the ununiform distribution of the electrodeposit hardly takes place; on the contrary, in the simultaneous electrolytic step, the spent nuclear fuel placed in the bottom of the crucible functions as the anode, so that the distance between the electrodes is not maintained constant. Consequently, the current density distribution on the surface of the cathode becomes ununiform, resulting in the ununiform distribution of the electrodeposit. Further, the distance between the lower end of the cathode and the surface of the spent nuclear fuel becomes shorter, the current density around the lower end of the cathode is thereby increased and accordingly the electrodeposit is concentrated around the lower end of the cathode, so that when stirring is not sufficiently conducted, the ions in the bulk region become insufficient and the processing speed is degraded.
Additionally, because the environment involved is highly corrosive owing to the use of chlorine gas, a material prepared by coating (with vapor deposition) graphite blank with pyrographite excellent in corrosion resistance is used as the material for the crucible doubling as the anode. However, because of the operation condition, such as high temperature molten salt and chlorine gas conditions, the operation life time of the crucible is in the order of 1,000 hours. Consequently, the crucible needs to be replaced at frequent intervals, leading to the decreasing of the processing speed.
Furthermore, it is conceivable that the electrolytic apparatus can be made larger in size as a measure for improving the processing speed. However, it is difficult to make a crucible made of pyrographite larger in size from the viewpoint of product fabrication.
Even when the constitution is such that the anode and cathode are installed in the crucible, the distance between the electrodes are not uniform, and hence the current density distribution on the cathode surface becomes ununiform, and the ununiform distribution of the electrodeposit takes place.
The bonding force between uranium oxide and plutonium oxide deposited as the forms of oxides and the surface of the electrodes are lower than the bonding force for the metallic state as in plating and the like. Consequently, in the conventional technique, in any case where the electrodeposit is concentrated in a particular portion, the possibility that the electrodeposit falls down from the surface of the cathodes during the electrolytic operation becomes high owing to the stirring effect of the various process gases blown into the molten salt.
Additionally, in view of the prevention of the criticality, it can hardly be an appropriate countermeasure to simply make the electrolytic apparatus larger in size.